We are about to concrete a new foundation,the building inspector has specified clay boards be used to counteract heave in our clay soil,when he inspected and passed the footings yesterday which are about 1.8m deep with a 150mm strip of greensand at the base,i asked how to place the boards in the trench,he said to pin them to the inside face of trench 500mm from the bottom of trench,surely this is too high?